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Timeline of Earth's geological history
In recent years, understanding of Earth's geological history has undergone significant advances, yet there is still much controversy over Earth's origins and the origins and evolution of life over time. This timeline will be based upon some of the most widely accepted theories, but as knowledge continues to advance, changes in our understanding of our planet's history are a certainty. Many alternative theories, particularly concerning evolution and phylogenetic relations, have been proposed; some of which will be discussed below. Hadean Eon Earth's origins are still a matter of significant scientific debate and philosophical conflict. The information provided below is based on one often-referenced theory, but, as previously mentioned, numerous alternate theories exist. 5.0 billion years ago (BYA) *The solar system forms. Gravity molds gas, fine dust, and clumps of rock into larger collections; these are eventually consolidated into spherical bodies. *Extreme heat and pressure at the core of the central mass triggers nuclear fusion; the mass becomes the sun. Most of the gas in the young solar system is assimilated into the sun; the rest eventually forms the gaseous planets (Jupiter, Saturn, Uranus, and Neptune) or is cast out of the solar system and into space. *The smaller, rocky lumps form the terrestrial planets (Mercury, Venus, Earth, and Mars), moons, asteroids, and comets. 4.5 BYA *Gravity compresses the terrestrial planets, including Earth, into hard spheres. There is some evidence that Earth's surface was primarily coated in magma at this time, and cooled over the following 100 million years. *Plate tectonics trigger a surge in volcanic activity on Earth. A toxic atmosphere of carbon dioxide, water vapor, and sulphur dioxide is formed. Ca. 4 BYA *An asteroid or dwarf planet, estimated to be between 500 and 650 kilometres in diameter, collides with Earth and disintegrates. The heat of the impact vaporizes the few scattered pockets of liquid water on Earth; debris from the crust of Earth and fragments of the smaller object are launched into orbit, where it is molded into Earth's moon. *In orbit, a mixture of fine ash and miscellaneous debris from the impact blocks sunlight from reaching Earth, which has a significant cooling effect on the planet. Atmospheric water vapour condenses and falls to Earth as rain. The moon's gravitational pull on Earth, coupled with Earth's own gravity, draws the water to the lowest points, forming the oceans. Archaean Eon Throughout much of the Archaean eon, Earth was still toxic and hostile to life. Yet it was in these conditions that life began. 3.8 BYA *Earth's oceans contain significant quantities of organic material, including the alkaline compound methylamine, which bonds with the dissolved mineral acids in the oceans, raising the pH enough for Earth's waters to be hospitable to life. *Nucleic acids, such as DNA and RNA, form from smaller carbon chain molecules, phosphates, and oxygen. These form the basis for life. Between 3.8 and 3.5 BYA *The first living cells, similar to modern-day bacteria, form, likely on the ocean floor near "black smoker" vents. *These earliest cells are accompanied by "near-living" biological agents such as viruses, some of which parisitize bacterial cells. 3.5 BYA *Cyanobacteria , the first photosynthesizing organisms, evolve, and form stromatolites . *Oxygen gas produced as a byproduct of photosynthesis by cyanobacteria begins to fill the oceans, and eventually "seeps" into the atmosphere. Category:Geological time periods